Formation of Nitrogen Bubbles During Solidification of Duplex Stainless Steels
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DUPLEX stainless steels are defined as solid solutions that contain both the ferrite phase and the austenite phase, the existence of which in a single microstructure give the material the properties of both the ferritic stainless steel and the austenitic stainless steel.[1] Nitrogen in steels is present as interstitial atoms and plays a major role in improving the pitting resistance, maintaining a good austenite stability as well as reducing the final steel price by replacing expensive Ni.[2–4] Therefore, the study of high nitrogen duplex stainless steels is of great significance for practical applications, such as aviation, building material, ultrasonic device and so on.
KAIJU DAI, BO WANG, FEI XUE, SHANSHAN LIU, JUNKAI HUANG, and JIEYU ZHANG are with the State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200072, P.R. China and also with the School of Materials Science and Engineering, Shanghai University. Contact e-mail: bowang@ shu.edu.cn Manuscript submitted October 3, 2017.
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However, when the nitrogen content is high, pores can form during the continuous casting process. These pores can seriously affect the quality of duplex stainless steels.[5] The subsequent gas pore improvement process cause economic losses. In order to improve the quality of duplex stainless steels and reduce costs, it is important to analyze the formation mechanism of bubbles during solidification. The results showed that formation of nitrogen bubbles during solidification was closely related to the nitrogen solubility, partial pressure, initial nitrogen content and cooling rate.[6] Arola and Wendt et al.[7] reported that the solubility of nitrogen in ferrite was lower than in austenite, and it was easy to form nitrogen bubbles when nitrogen segregation exceeded nitrogen saturated solubility in ferrite for duplex stainless steels during solidification. Ridolfi et al.[8] suggested that the larger the local secondary dendrite arm spacing during solidification, the smaller the pressure was required to form bubbles, and small pores located just below the slab surface arose when the nitrogen content was greater than about 0.2 wt pct. Seong-ho Yang et al.[9] argued that when the nitrogen content exceeded 0.19 wt pct at atmospheric pressure, nitrogen bubbles were precipitated
inside the ingot, and the higher the nitrogen content, the easier it was to precipitate nitrogen bubbles. Especially, Kangwei Li et al.[10] qualified the homogeneous and heterogeneous nucleation of bubbles in molten steel on reduced pressure conditions, and obtained the formula for the critical radius of bubble nucleation, and argued that the critical sizes of bubble nucleation were closely related to surface tension of liquid steel, depth, partial pressure, steel density and so on. And the critical radius of bubble nucleation was 0 to 100 lm when the depth of molten steel was 0.32 to 1.03 m, the critical radius of bubble nucleation was greater than 100 lm when the depth of molten steel was 1.03 to 1.40 m. How
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